If what God says he is going to do for his people seems marvellous, that is, too wonderful to be believed, then let them think again. With great expectation comes potentially great disappointment if we fail to receive what we expected.
How the Lord loves his people to be encouraged! He speaks as a father who has a wonderful gift to give to his child, but one that is so wonderful that it is liable to stretch the credibility of the child beyond breaking point, and that defeats the purpose of it, for the father wants the child to enjoy the gift even before it is given. This is such a gentle mode of speaking. God does not take issue with them for finding it hard to believe him, but he almost picks them up and lifts them over the barrier.